I have posted this under a separate thread to get it away from the GL ranting from which the discussion of regauging shays originated. Anyway, these are my thoughts originating from some previous studies of the behavior of universal joints. I have never worked on or with shays. I am not an expert. I did not stay at a Holiday Inn Express last night. I am not a doctor nor do I play one on TV. But geometry and trigonometry are what they are and I have done my best to explain it. Flames may be directed into the firebox.
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When passing power through a universal joint, the geometry of the joint causes the output shaft to speed up and slow down twice per revolution. If the joint is straight, the speed oscillation is zero. The greater the bend in the joint the greater the speed oscillation. The magnitude of the oscillation increases exponentially (not linearly) with the size of the angle.
With two or more joints, the oscillation adds or subtracts. With two joints bent the same direction (within a given plane), the effect is additive. Joints bent the opposite direction subtract. Joints bent the opposite direction to the same angle (meaning the end shafts are parallel) completely cancel out. This is why universal joints are used in rear wheel drive autos (where the transmission output shaft is parallel with the rear axle input shaft) and not in front wheel drive cars (where the wheel could be at a variety of angles to the transmission). Look under a tall delivery or monster truck and you will see the input shaft of the rear axle always points forward even though the drive shaft angles down to it. Otherwise the effect would not be cancelled and the wheels would exhibit speed oscillation.
In a system with parallel end shafts, constant velocity in yields constant velocity out. However, this does not change the fact that the joints create speed oscillations (a sine curve, I believe) in the middle shaft. The more offset the end shafts are (thus greater angles in the joints), the more drastic the speed oscillations in the center shaft become. And not just speed. High torque at low speed alternates with low torque and high speed. This creates a hammering on the bearing surfaces of the universal joint. At small angles the hammering is small, and steep angles it is large. At some angle you cannot generate enough torque to transfer power through the joint.
As far as a Shay engine being mounted higher or further in or out, this will result in some hammering of the universals joint bearings, like it or not. Whether it is significant is a matter of opinion. Some offset is normal, as seen in RWD auto drivelines, and creates negligible wear.
Things get more complicated in curves. When going straight the orientation of the engine set relative to the driveline is (overall) compensated for by the double universal joint. When the truck starts turning, the angles are no longer the same in the two joints, or even in the same plane in all cases. The effect of the engine offset is still compensated for, but the additional angle means there will be some uncompensated speed oscillations down the driveline. As it passes more joints bent in the same direction, the effect will be increased. One joint or a hundred, the size of the oscillation depends on the overall angle from the point of constant speed. Note that this effect also applies to Climaxes and Heislers just as much as Shays, except that the net angle on the shay would be greater if turning right and smaller if turning left. The angle in Heislers and Climaxes would be constant at the median between the two shay angles.
The expansion joints play a small part in the universal joint activity. When turning left the center shaft becomes longer, thus the angles in the joints become smaller. Conversely during a right turn the center shaft becomes shorter and the angles larger. Note that this effect is exaggerated if the engine set is placed further out as in a narrow gauged shay. In this respect this is bad on a right turn and good on a left turn.
So, does an offset engine set cause elevated universal joint wear? Of course it does. However, so long as you are not way off or if you Shay has short shafts to begin with, it may be acceptable wear. There is probably worse hammering as a result of the angles of tight turning than of a slightly offset engine set. After all, hammering during turning can only be compensated for by slack in the driveline or slipping wheels. At the same time, the hammering due to curvature is periodic while the hammering due to engine alignment is constant.
Those are my opinons, anyway.
